1. Field of the Invention
The present invention relates to an S-shaped correction signal circuit having an S-shaped correction signal condenser, the capacitance of which is controllable, installed in a horizontal deflection circuit, and in particular to an S-shaped correction signal circuit having an S-shaped correction signal condenser the capacitance of which is controllable within a horizontal scan period interval.
2. Description of the Conventional Art
FIG. 1 illustrates a conventional horizontal deflection circuit. In the drawing, HD denotes a horizontal driving signal generator, T.sub.R denotes a horizontal output transistor, D denotes a damper diode, C.sub.r denotes an oscillation condenser, L.sub.DY denotes a horizontal deflection coil, C.sub.S denotes an S-shaped correction signal condenser, FBT denotes a flyback transformer, and +B denotes a horizontal deflection circuit voltage. Since the operation of the horizontal deflection circuit is well known in the industry, the description on the operation thereof will be omitted. Therefore, only the S-shaped correction signal condenser C.sub.S will be explained.
First, as shown by the solid line in FIG. 2A, the deflection current which flows at a deflection yoke ideally has a straight line characteristic. Actually, the deflection current has a non-straight line characteristic due to a resistance loss of the damper diode D, the On resistance loss of the horizontal output transistor T.sub.R, and an effective resistance value of the deflection yoke, etc., as shown in the drawing as a broken line.
The horizontal screen size (scan length) is increased in the front half portion of the scan deflection, and the same is gradually decreased in the rear half portion of the scan due to the above-described characteristics.
Therefore, scan lines on the left side of the screen are horizontally extended, and scan lines on the right side of the screen are horizontally shortened, namely, a screen distortion phenomenon occurs on both the sides of the screen.
Furthermore, since a Braun tube has a non-spherical surface, a non-straight line characteristic exists therein. Therefore, the width of a displayed image is decreased in the center portion of the screen, and the same is extended at the edge portions of the screen, namely, a distortion is formed in the screen image.
In order to correct the above-described distortion problem, an S-shaped correction signal condenser C.sub.S is provided, which causes the horizontal deflection signal to the oscillate in accordance with the inductance and capacitance values of the horizontal deflection circuit shown in FIG. 1, and the condenser C.sub.S forms a series LC circuit with the horizontal deflection coil L.sub.DY, so that an S-shaped correction (compensation) of the horizontal deflection signal is implemented. The correction is performed depending on the degree of distortion. For example, if an oscillation is performed at a frequency twice the horizontal sync. frequency, the correction current becomes an oscillation current as shown in FIG. 2B.
Therefore, it is possible to obtain a horizontal deflection current characteristic, as shown in FIG. 2C, by the correction current. Namely, in the front and end portions in the horizontal deflection current scanning interval, the current slant is decreased, and the amplitude in the horizontal direction is narrow, and in the center portion of the scanning interval, the current slant is increased, and the horizontal direction amplitude is widened.
Therefore, it is possible to decrease the distortion of the image which occurs due to the non-straight line characteristic of the deflection current and the non-spherical surface of the Braun tube.
In the above-described conventional S-shaped correction signal circuit, since the capacitance of the correction condenser C.sub.S is fixed, it is difficult to obtain a desired correction effect of a high accuracy, and in addition, it is difficult to control the capacitance of the correction condenser.